linux-ext4 - Re: [PATCHv2 1/1] ext4: Fix overflow case for map.m_len in ext4_iomap_begin

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Date:   Wed, 22 Apr 2020 17:56:05 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     linux-ext4@...r.kernel.org
Cc:     jack@...e.cz, tytso@....edu, adilger@...ger.ca,
        darrick.wong@...cle.com, hch@...radead.org,
        linux-fsdevel@...r.kernel.org,
        syzbot+77fa5bdb65cc39711820@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCHv2 1/1] ext4: Fix overflow case for map.m_len in
 ext4_iomap_begin_*

NACK on this patch.

Even though this fixes syzcaller reproducer, still it seems the right
fix should be:-
1. To make EXT4_MAX_LOGICAL_BLOCK to 0xfffffffe
2. And also add fiemap_check_ranges() call in overlayfs.
This is because fiemap_check_ranges() takes care of truncating the
length parameter to max sb->s_maxbytes which underlying filesystem can
handle. This will be similar to how VFS calls for fiemap on underlying
FS.

Currently running xfstests on patches which implements above logic.

-ritesh

On 4/17/20 12:22 AM, Ritesh Harjani wrote:
> EXT4_MAX_LOGICAL_BLOCK - map.m_lblk + 1 in case when
> map.m_lblk (offset) is 0 could overflow an unsigned int
> and become 0.
> 
> Fix this.
> 
> Fixes: d3b6f23f7167 ("ext4: move ext4_fiemap to use iomap framework")
> Reported-and-tested-by: syzbot+77fa5bdb65cc39711820@...kaller.appspotmail.com
> Reviewed-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> ---
> @Jan,
> I retained your Reviewed by, since there was no logic change, but just couple
> of minor change - missed semicolon and tab space issue.
> 
>   fs/ext4/inode.c | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e416096fc081..d9feaaad8ab8 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3424,6 +3424,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>   	int ret;
>   	struct ext4_map_blocks map;
>   	u8 blkbits = inode->i_blkbits;
> +	loff_t len;
>   
>   	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
>   		return -EINVAL;
> @@ -3435,8 +3436,11 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>   	 * Calculate the first and last logical blocks respectively.
>   	 */
>   	map.m_lblk = offset >> blkbits;
> -	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
> -			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> +	len = min_t(loff_t, (offset + length - 1) >> blkbits,
> +		    EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> +	if (len > EXT4_MAX_LOGICAL_BLOCK)
> +		len = EXT4_MAX_LOGICAL_BLOCK;
> +	map.m_len = len;
>   
>   	if (flags & IOMAP_WRITE)
>   		ret = ext4_iomap_alloc(inode, &map, flags);
> @@ -3524,6 +3528,7 @@ static int ext4_iomap_begin_report(struct inode *inode, loff_t offset,
>   	bool delalloc = false;
>   	struct ext4_map_blocks map;
>   	u8 blkbits = inode->i_blkbits;
> +	loff_t len;
>   
>   	if ((offset >> blkbits) > EXT4_MAX_LOGICAL_BLOCK)
>   		return -EINVAL;
> @@ -3541,8 +3546,11 @@ static int ext4_iomap_begin_report(struct inode *inode, loff_t offset,
>   	 * Calculate the first and last logical block respectively.
>   	 */
>   	map.m_lblk = offset >> blkbits;
> -	map.m_len = min_t(loff_t, (offset + length - 1) >> blkbits,
> -			  EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> +	len = min_t(loff_t, (offset + length - 1) >> blkbits,
> +		    EXT4_MAX_LOGICAL_BLOCK) - map.m_lblk + 1;
> +	if (len > EXT4_MAX_LOGICAL_BLOCK)
> +		len = EXT4_MAX_LOGICAL_BLOCK;
> +	map.m_len = len;
>   
>   	/*
>   	 * Fiemap callers may call for offset beyond s_bitmap_maxbytes.
>